1. Field of the Invention
The present invention relates to a process for optically resolving optically inactive 2-hydroxy-4-phenylbutanoic acid to obtain an active compound thereof.
2. Description of the Prior Art
(.+-.)-2-Hydroxy-4-phenylbutanoic acid is an important starting compound for synthesis of angiotensin converting enzyme-inhibiting pharmaceuticals such as Cilazapril, Benzapril, Quinapril, Enalapril, Ramipril, Lisinopril and 2-hydroxy-4-phenylbutanoic acid as prepared by chemical synthetic methods is generally obtained as an optically inactive racemate, i.e. (.+-.)-carboxylic acid. However, pharmaceuticals derived from (.+-.)-2-hydroxy-4-phenylbutanoic acid contained in (.+-.)-2-hydroxy-4-phenylbutanoic acid have a much poorer inhibiting ability than that of pharmaceuticals derived from (-)-2-hydroxy-4-phenylbutanoic acid, and thus the development of a technique has hitherto been desired for effectively optically resolving (.+-.)-2-hydroxy-4-phenylbutanoic acid to obtain highly pure (-)-2-hydroxy-4-phenylbutanoic acid.
Heretofore, as a method of obtaining an optically active 2-hydroxy-4-phenylbutanoic acid has been reported a method of obtaining the optically active compound by treating (.+-.)-2-hydroxy-4-phenylbutanoic acid with an optically active method to form menthyl esters, repeating recrystallization from petroleum ether to obtain an optically active menthyl ester and hydrating it (Optical Resolution Procedures for Chemical Compounds pp. 476 (1981) published by OPTICAL RESOLUTION INFORMATION CENTER, Manhattan College, Riverdale, New York).
However, since the resolving agent used in this method is comparatively expensive and moreover the yield is not high of an optically active 2-hydroxy-4-phenylbutanoic acid obtained by resolution, there has been a problem that an optically active 2-hydroxy-4-phenylbutanoic acid cannot cheaply be supplied thereby.